The present invention relates to a CMOS (Complementary Metal Oxide Semiconductor) image sensor; and, more particularly, to a pixel array of the CMOS image sensor and a method for driving the pixel array.
Generally, an image sensor is an apparatus to capture images using light sensing semiconductor materials. Since brightness and wavelength of light from an object are different in their amount according to the reflection area, electrical signals from pixels are different from one another. These electrical signals are converted into digital signals, which can be processed in a digital circuit, by an analogue-to-digital converter. Thus, the image sensor needs a pixel array having tens to hundreds of thousands of pixels, a converter for converting analogue voltages into digital voltages, hundreds to thousands of storage devices and so on.
Referring to FIG. 1, a conventional CMOS image sensor includes a control and interface unit 10, a pixel array 20 having a plurality of CMOS image sensing elements, and a single slope AD converter 30. The single slope AD converter 30 also includes a ramp voltage generator 31 for generating a reference voltage, a comparator (operational amplifier) 32 for comparing the ramp voltage with an analogue signal from the pixel array 20 and a double buffer 40.
The control and interface unit 10 controls the CMOS image sensor by controlling an integration time, scan addresses, operation modes, a frame rate, a bank and a clock division and acts as an interface with an external system. The pixel array 20 consisting of Nxc3x97M unit pixels having excellent light sensitivity senses images from an object. Each pixel in the pixel array 20 includes a transfer transistor, a reset transistor and a select transistor. The single slope AD converter 30 converts analogue signals from the pixels array 20 into digital signals. This AD conversion is carried out by comparing the ramp voltage with the analogue signals. The comparator 32 searches for a point at which the analogue signals are the same as the falling ramp voltage with a predetermined slope. When the ramp voltage is generated and then starts falling, the control and interface unit 10 generated count signals to count the degree of the voltage drop. For example, the ramp voltage starting the voltage drop, the converted digital value may be xe2x80x9c20xe2x80x9d in the case where the analogue signals are the same as the falling ramp voltage at 20 clocks of the control and interface unit 10. This converted digital value is stored in the double buffer 40 as digital data.
Where the CMOS image sensor supports the correlated double sampling (hereinafter, referred to as a CDS) in order to generate images of high quality, unit pixels 100 and 120 in the pixel array include a photodiode and four transistors, respectively, as shown in FIG. 2. Also, the four transistors in the unit pixel 100 include a transfer transistor M21, a reset transistor M11, a drive transistor M31 and a select transistor M41. The transfer transistor M21 transfers photoelectric charges generated in the photodiode 101 to sensing node A, the reset transistor M11 resets sensing node A in order to sense a next signal, the drive transistor M31 acts as a source follower and the select transistor M41 outputs the digital data to an output terminal in response to the address signals.
In accordance with the CDS, the unit pixel 100 obtains a voltage corresponding to a reset level by turning on the reset transistor M11 and turning off the transfer transistor M21. Also, the unit pixel 100 obtains a data level voltage by turning off the transfer transistor M21 in a turn-off state of the reset transistor M11 and reading out photoelectric charges generated in the photodiode 101. An offset, which is caused by the unit pixel 100 and the comparator 32, may be removed by subtracting the data level from the reset level. This removal of the offset is essential to the CDS. That is, by removing an unexpected voltage in the unit pixel 100, it is possible to obtain a net image data value.
FIG. 3 shows a timing chart illustrating control signals to control transistors of the unit pixel shown in FIG. 2. The operation of the unit pixel 100 will be described with reference to FIG. 3.
1) In section xe2x80x9cAxe2x80x9d of FIG. 3, the transfer transistor M21 and the reset transistor M11 are turned on and the select transistor M41 is turned off, so that the photodiode 101 is fully depleted.
2) In section xe2x80x9cBxe2x80x9d, the turned-on transfer transistor M21 is turned off, so that the photodiode 101 receives light from an object, generates photoelectric charges and integrates the photoelectric charges (Section xe2x80x9cBxe2x80x9d continues on regardless of the states of the reset transistor M11 and the selector transistor M41, until the transfer transistor M21 is again turned on).
3) In section xe2x80x9cCxe2x80x9d, the reset transistor M11 and the transfer transistor M21 keep on a turn-on state and a turn-off state, respectively, and the select transistor M41 is turned on, so that reset voltage level is outputted through the select transistor M41 and the drive transistor M31 driven by the voltage level at sensing node A.
4) In section xe2x80x9cDxe2x80x9d, the resent transistor M11 is turned off and then the reset voltage level generated in section xe2x80x9cCxe2x80x9d is settled.
5) In section xe2x80x9cExe2x80x9d, the reset voltage level of section xe2x80x9cDxe2x80x9d is sampled.
6) In section xe2x80x9cFxe2x80x9d, the reset transistor M11 and the select transistor M41 keep on a turn-off state and a turn-on state, respectively, and the transfer transistor M21 is turned on, so that a data voltage level corresponding to photoelectric charges integrated in the photodiode 101 during the time of section xe2x80x9cBxe2x80x9d, is transferred to the output terminal through the sensing node A, the drive transistor M31 and the select transistor M41.
7) In section xe2x80x9cGxe2x80x9d, the transfer transistor M21 is turned off and then the data voltage level generated in section xe2x80x9cFxe2x80x9d is settled.
8) In section xe2x80x9cHxe2x80x9d, the data voltage level of section xe2x80x9cGxe2x80x9d is sampled. The reset voltage level and the data voltage level sampled in sections xe2x80x9cExe2x80x9d and xe2x80x9cFxe2x80x9d, respectively, are outputted to the AD converter 30 (FIG. 1) and converted into two digital signals. The difference of two digital signals becomes an output image value of the CMOS image sensor with respect to an image inputted from the photodiode 101 (FIG. 1).
This conventional unit pixel employs four transistors per pixel in order to support the CDS, thus increasing the chip size of the CMOS image sensor.
It is, therefore, an object of the present invention to provide a CMOS image sensor that may reduce its chip area by decreasing the number of transistor for a pixel array and a method for driving the CMOS image sensor.
In accordance with an aspect of the present invention, there is provided a CMOS (Complementary Metal Oxide Semiconductor) image sensor, comprising: a unit pixel, the unit pixel including: a plurality of photodiodes receiving incident light form an object for generating photoelectric charges; a plurality of transferring means corresponding to the plurality of photodiodes, for transferring the photoelectric charges from the plurality of photodiodes to a single sensing node in response to control signals from an external controller, wherein the control signals controls the plurality of transferring means so that the photoelectric charges from the plurality of photodiodes are selectively transferred to the single sensing node; a common reset means for resetting the single sensing node, wherein the reset means determines reset levels of the single sensing node corresponding to each of the plurality of photodiodes; and a common outputting means for outputting electrical signals corresponding to voltage levels of the single sensing node, wherein the CMOS image sensor samples the electrical signals through the correlated double sampling and then outputs a final image value to the external device.
In accordance with another aspect of the present invention, there is provided a method for driving a CMOS image sensor to obtain a single outputted by the correlated double sampling from photoelectric charges generated in a plurality of photodiodes using a single sensing node, wherein the plurality of photodiodes are electrically coupled to the single sensing node, the method comprising the steps of: (a) generating the photoelectric charges in each photodiode; (b) resetting the single sensing node and obtaining a first electrical signal from the single sensing node; (c) transferring the photoelectric charges from one of the photodiodes to the single sensing node and then obtaining a second electrical signal from the single sensing node; (d) resetting the single sensing node and obtaining a third electrical signal from the single sensing node; and (e) transferring the photoelectric charges from another of the photodiode to the single sensing node and then obtaining a fourth electrical signal from the single sensing node.
In according with further another aspect of the present invention, there is provided a unit pixel in a CMOS (Complementary Metal Oxide Semiconductor) image sensor, comprising: a first photodiode for receiving light from an object and for generating and integrating photoelectric charges; a first transfer transistor coupled between the first photodiode and a single sensing node, for transferring the photoelectric charges generated in the first photodiode to the single sensing node, in response to a first control signal; a second photodiode for receiving light from the object and for generating and integrating photoelectric charges; a second transfer transistor coupled between the second photodiode and the single sensing node, for transferring the photoelectric charges generated in the second photodiode to the single sensing node, in response to a second control signal; a reset transistor coupled between a power supply and the single sensing node, for outputting the photoelectric charges stored in the single sensing node, in the response to a third control signal; a drive transistor coupled to the power supply, for acting as a source follower in response to an output of the single sensing node; and a select transistor coupled to the drive transistor, for outputting an image data driven by the drive transistor in response to address signals.
In accordance with still another aspect of the present invention, there is provided a method for driving a unit pixel which comprises a first photodiode for receiving light from an object and for generating and integrating photoelectric charges; a first transfer transistor coupled between the first photodiode and a single sensing node, for transferring the photoelectric charges generated in the first photodiode to the single sensing node, in response to a first control signal; a second photodiode for receiving light from the object and for generating and integrating photoelectric charges; a second transfer transistor coupled between the second photodiode and the single sensing node, for transferring the photoelectric charges generated in the second photodiode to the single sensing node, in response to a second control signal; a reset transistor coupled between a power supply and the single sensing node, for outputting the photoelectric charges stored in the single sensing node, in the response to a third control signal; a drive transistor coupled to the power supply, for acting as a source follower in response to an output of the single sensing node; and a select transistor coupled to the drive transistor, for outputting an image data driven by the drive transistor in response to address signals, the method comprising the steps of: (a) fully depleting the first and second photodiodes; (b) receiving light in the first and second photodiodes and generating photoelectric charges; (c) turning on the reset transistor, turning off the first and second transfer transistors and outputting a reset voltage level through the single sensing node, the drive transistor and the select transistor; (d) turning off the reset transistor, turning on the first transfer transistor and outputting a data voltage level of the photoelectric charges generated in the first photodiode through the single sensing node, the drive transistor and the select transistor; (e) turning on the reset transistor, turning off the first and second transfer transistors and outputting the reset voltage level through the single sensing node, the drive transistor and the select transistor; and (f) turning off the reset transistor, turning on the second transfer transistor and outputting a data voltage level of the photoelectric charges generated in the second photodiode through the single sensing node, the drive transistor and the select transistor.